MB4 261a: Making Sure the Eruv is Accessible (394:2-3); Bracha on Shituf Eruv (395:1)

The eruv chatzerot food needs to be in a place that's halachically accessible on Shabbos for at least some point during bein hashmashot (e.g. if it's in a building that collapsed and it can only be accessed via digging with a shovel it's invalid).

If its put in place before bein hashmashot but it was eaten during bein hashmashot.

Using the eruv chatzerot food for another mitzvah (e.g. lechem mishna). Our practice is to use matzah and make an eruv chatzerot once a year.

What if the eruv gets lost or you forget where it is located or becomes inaccessible?

Making a shituf mavuyot is a mitzvah because it prevents people from violating הוצאה on Shabbos. (Eruv techumin by contrast is reshut, not a mitzvah).

What are we making a bracha on? That we were given the mitzvah of eruv via which we are permitted to carry on Shabbos.

When is the bracha made?